From what little I picked up in a weeks gliding course....
Much more important than the actual minimum sink or best glide speed of the aircraft is the vertical movement of the air you're flying in. Because of this, gliders are fitted with a MacCready ring around the VSI. This tells you the speed to fly given the sink speed of the air and is based on the polar diagram for the aircraft. In other words: the ring is tailored for each aircraft type.
The idea behind it is that you want to leave an area of sinking air as soon as possible, but the faster you fly the more inefficient your flight becomes. So it's a tradeoff between the amount of time you fly in the sinking air vs. the sink you incur with the aircraft itself by flying fast.
Wikipedia has the full explanation:
Speed to fly - Wikipedia, the free encyclopedia
So...
If best glide speed means that the glide range is 5 miles from 2500 ft then what is impact of increasing/decreasing this speed by say 10 knots???
Depending on the vertical speed of the air over that five mile range and how the pilot handles it, the glider might be on the ground well before that five miles, or be at 3500ft or even higher. And his cruise speed might have been varied between 80 and 160 kph or even more.
Oh, and gliders don't fly in non-thermal conditions. Not for long anyway. (Ridge lift and such excepted.)